Phase modulation and amplitude modulation receiving system



Sept. 12, 1944. v CROSBY" PHASE MODULATION AND AMPLITUDE MODULATION RECEIVING SYSTEM Filed Feb. 18, 1942 4 Sheets-Sheet 1 .555 MN RQRYSQ I v mQEBQS J vwrLr.

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' 1 PHASE MODULATION AND AMPLITUDE MODULATION RECEIVING SYSTEM Fil ed Feb. 18, 1942 1 4 Sheets-Sheet 3 & N I

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flll Q H nn uu It I!" k a -\QQQQQQ, N 4 g x g :55 INVENTOR 3i 3, r L Mame/1y 6 (Pas/an. I k l N u a 3 BY E E a g 8 k O u ATTORNEY Sept 12, 1944.

M. G. CROSBY PHASE MODULATION AND AMPLITUDE MODULATION RECEIVING SYSTEM Filed Feb. 18, 1942 To HM. cum/7' AFC AFC

4 Sheets-Sheet 4 Maze/1v G Ceasav ATTORNEY Patented Sept. 12, 1944 PHASE MODULATION AND AMPLITUDE MODULATION RECEIVING SYSTEM Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 18, 1942, Serial No. 431,326

9 Claims. (Cl. 250-27) My present invention relates to receiving systems of the type adapted to receive phase modulated carrier waves or amplitude modulated carrier waves, and more particularly to receivers of the type utilizing a crystal filter network functioning to provide carrier exaltation and conversion of the phase modulated waves to amplitude modulated waves.

There has been described in the Proceedings of the Institute-of Radio Engineers, published in the February, 1939, issue thereof (pages 128, 129 and 130), a receiver of the type wherein a crystal filter is employed in a. phase modulation receiver to convert the phase modulated waves into amplitude modulated waves; the crystal filter further functioning to provide carrier exaltation.. In that receiver the crystal filter utilizes under-neutralization combined with over-neutralization in a manner described in the aforesaid publication. It is, also, described in the aforesaid publication that the crystal filter may be utilized in connection with amplitude modulated carrier waves thereby pro.- viding carrier exaltation, the detector for the amplitude modulated carrier waves in that case being independent of the rectifier for the converted phase modulated carrier waves. Reference is, also, made to my following patents showing phase modulated (PM) and amplitude modulated (AM) receivers wherein overand underneutralized crystal filters areemployed: U. S. Patent 2,204,575, granted June 18, 1940; U. S. Patent 2,156,376, granted May 2, 1939; and U. S.

Patent 2,192,684, granted Mar. 5, 1940.

Special reference is made to my pending application', Serial No. 336,316 filed May 21, 1940, wherein is shown a system of the type disclosed in the present application,

Among the various objects of the present invention is to provide various embodiments of PM and AM receivers utilizing crystal filters employing a four electrode crystal holder connected in a manner to provide an under-neutralized output from one output electrode and an over-neutralized output from the other output electrode whereby back-to-back carrier exaltation may be secured at the detector network.

still other objects of my invention are to provide various forms of PM-AM receivers employing crystal filters of the over and under-neutralized type, wherein the piezo-electric crystal element is positioned in a predetermined manner in a four electrode crystal holder.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference '0 the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into efiect.

In the drawings:

Fig. 1 shows one embodiment of the invention,

Figs. 2a and 2b show the characteristics of the crystal filter,

Figs. 3a to 3d inclusive illustrate graphically the functioning of the crystal filter,

Fig. 4 shows another, and modified, embodiment of the invention,

Fig. 5 illuztrates a further embodiment,

Fig. 6 illustrates a simplified embodiment of the invention,

Fig. 7 illustrates a modific tion of the arrangement in Fig. 6.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 that portion of a superheterodyne receiver of the phase modulation-amplitude modulation type which is essential to a proper understanding of the invention. The abbreviations PM for phase modulation and AM for amplitude modulation will hereinafter be used for brevity. The networks prior to the intermedate frequency (I. F.) transformer 4 are not necessary to, a proper understandong of this invention, and, therefore, can be generally referred to. It will be understood that they include the signal collector circuit which is followed by the usual converter whose input circuit may be tuned to the desiredcarrier frequency. It will be further understood that there may be applied to the tunable prior to the transformer 4 to bring the carrier frequency of the modulated carrier energy to The converter is a local oscillator It will be understood that there may be employed suflicient heterodyning" thatfrequency value which is equal to the operating I. F. value of transformer 4. This is true regardless of whether PM or AM signals are being collected at the signal collector.

is deviated in accordancewith the modulation amplitude, but the frequency deviation at the higher modulation frequencies is emphasized. Damping resistors are shunted across each of the tuned circuits of tranformer l in order to provide a sufiiciently wide pass band so that the widest frequency swing of the PM waves may be transmitted with eiiiciency' to the crystal filter network. The crystal filter comprises a piezoelectric crystal I which is tuned to the I. F. value. One end of the crystal I is positioned. between a pair of opposed electrodes 2 and 3, the latter being connected to the opposite ends of the tuned secondary circuit. The opposite end of crystal I is positioned between the crystal elecof resistor I2. AFC bias for the reactance tube,

which has already been referred to at a prior point, is derived from the cathode end of re when such,waves are transmitted through the trodes 5 and 6. It will, therefore, be seen that the crystal ispositioned in a four electrode crystal holder. The electrodes land 5 are connected by the coupling capacity 01, while the electrode 6 is connected to the high potential side of the transformer secondary circuit by coupling condenser C'z.

The tubes I and 8 are driver tubes which function to feed the modulated waves to the difierential diode detection network. The control grid of tube I is connected to the electrode 5, while the control grid of tube 8 is connected to the electrode 6. Normal control grid bias is provided for the control grids I and 8 in the usual manner as shown in Fig. 1. The plates of the driver tubes are connected to the primary tuned cirreceiving system, a pair of parallel-connected infinite-impedance detectors are also fed by the driver tubes 1 and 8'. Thus, tubes I4 and I5 have their control grids coupled to the opposite sides of the secondary tuned circuits of I. F. trans-- formers I0 and I I. The cathodes of tubes I4 and I5 are arranged so that their detected outputs are taken from series connected load resistors I4 and I5 by means of condensers Ca and C4. The control grid of each of the tubes I4 and I5 is connected to the junction of resistors I4 and I5 through a respective grid-return resistor. The junction of the load resistors I4 and I5 is grounded, and each of the load resistors is shunted'by a condenser which bypasses the I. F. current but not the modulation currents. In this way each of tubes III and I5 functions as a degenerative plate circuit detector, or what has also been described as an infinite-impedance diode detector. It will be seen that the plates of tubes I 4 and I5 are connected to positive potential points of a direct current source. Condenser C5 and resistor R form an equalizing circuit which.

attenuates the lower audio modulation frequencies to compensate for the exaltation of these frequencies which is effected by the crystal filter during amplitude modulation reception. The high potential ends of load resistors I I and I 5' are coupled through the modulation coupling condensers C3 and C4 to the input terminal of the equalizing condenser C5.

The crystal filter functions to convert the PM waves into AM waves so that the latter may be detected in the diiferential diode detection network. It has been found that when a crystal cuits of a pair of I. F. transformers I0 and II The transformer III has its primary circuit tuned to the operating I. F. value, while the primary circuit of transformer II is also tuned to theoperating I. F. value. Each of these primary circuits is shunted by a damping resistor to provide the proper band pass. characteristic. The junction of the last mentioned primary tuned circuits is connected to the positive terminal of a direct current source (not shown).

The diodes I2 and I3 are arranged in a diiferential detection circuit for the detection of the phase modulated I. F. waves. The diode I2 has its anode connected to the high potential side of the secondary tuned circuit of transformer III. It will be understood that the secondary circuit is tuned to the I. F. value, while the cathode of diode I2 is connected to the low potential side of the secondary circuit through the load resistor I2. In the same way the diode I3 has its load resistor [3' connected in series with load resistor I2, the anode of diode I3 being connected to the tuned secondary circuit of transformer I I. formers III and II are connected in series hetweenthe anodes of diodes I2 and I3, while the junction of the secondary circuits is connected to the junction of load resistors I2 and I3. Each of the resistors I2 and I3 is shunted by an I. F. bypass condenser. The cathode end of load resistor I3 is grounded, and the phase modulation voltage is taken off from the cathode end The secondary circuits of transfilter of the type in which the holder capacitance is neutralized is operated inthe off-neutralized condition, it'is capable of converting PM waves into AM waves. The conversion is effected by shifting the phase of the modulation side bands with respect to the carrier.

Figs. 3a to 3d show the manner in which the crystal filter functions to perform the conversion for a given instant of time. The vector relations between the carrier and side frequencies in an AM wave are as shown in Fig. 3a. The carrier is denoted by the vector C and the upper and lower modulation side bands are denoted by the letters U and L respectively. The phase relation of the three components is such that as theside band frequencies rotate with respect to the carrier vector, they combine with the carrier in a manner to add and subtract from the carrier amplitude and thereby vary the resultant amplitude sinusoidally.

The relation of the carrier and side frequencies in a PM wave for a given instant of time is shown in Fig. 3b. It'will be seen that the carrier .of the PM wave is shifted degrees with respect to that of the AM wave. In view of this phase relation existing between the carrier and side frequencies of a PM wave, it can be seen that the PM wave can be converted into an AM wave for subsequent detection by phase shifting the carrier with respect to the side bands, or by phase shifting the side bands with respect to the carrier.

Normally, if there were no capacity coupling between electrodes 2 and 5 of the crystal filter the filter characteristic obtained between electrodes 23.and -6 would bega simple "resonance curve. However, with the capacity coupling effected by condenser C1, an' over-neutracharacteristic obtained from electrode 8 may be under-neutralized by means of condenser C2. Since there is a 180 degree phase reversal between electrodes 5 and 8, the modulated wave energy coupled from the high side of the transformer secondary to electrode 6'by condenser C2 produces an under-neutralizing effect, whereas the energy transmitted through condenser C1 produces an over-neutralizing effect.

Figs. 2a. and 2b respectively show the underneutralized characteristic and the over-neutralized characteristic. The under-neutralized characteristic causes the PM wave represented by Fig. 3b to be converted to the vector relations shown in Fig. 3c. The side frequencies in the vicinity of the rejection frequency in Fig. 2a. are substantially eliminated so that for these lower modulation frequencies the phase modulation is converted to amplitude modulation by the removal of one side band. The side frequencies in the immediate vicinity of the midband, or center, frequency are exalted with the carrier so that an increased output might be expected from these modulation frequencies. On the other hand, due to the over-neutralized characteristic due to condenser C1, the PM wave is converted into an AM wave as shown in Fig. 3d. It will be understood that in this case the side bands are shifted 90 degrees with respect to the carrier, but in a direction opposite .to that effected by the under-neutralized characteristic. The amplitude envelopes are 180 degrees out of phase, and the detected outputs'must, therefore, be combined with a 180 degrees phase reversal between them so as to make the outputs additive. This combination cancels amplitude modulation present on the applied PM wave. The phase reversal is effected by grounding the cathode of diode i3, and making the other cathode the high potential point.

Resistors R, and R1 form output impedances which can be varied to obtain different degrees of selectivity from the two crystal filters. The condensers C1 and C2 are shown as actual condensers which may aid or oppose the natural holder capacities. I have found that the natural holder capacities can be adjusted by properly positioning the metal electrodes 2 and 3, 5

and 6 so as to obtain the desired capacities necessary to provide the coupling capacities C1 and C2 without the addition of external condensersx Since these electrode plates are near to each other they have an appreciable natural capacity. By properly proportioning thesizesof metal electrodes 2 and 5, the natural capacity between made equal to the capacity of C2 thereby-mak-.

ing it possible to dispense with an external condenser between plates 2 and 6. For this type 01 crystal holder a simple filter is provided which A feature common to the system of Fig. 1, and the circuits of Figs. 4 to 7, is the four-electrode crystal filter. This filter is novel in that an over-neutralized and an under-neutralized filter output may be obtained without the requirement of a push-pull tuned transformer feeding the filter. One side of the secondary of transformer l is grounded, whereas in my prior circuits shown in the aforesaid Patent No. 2,204,575 itv was midtapped. The reason for the push-pull secondary of the last-mentioned' prior circuits is to obtain opposite-phase energy for neutralizing. In the present circuits, common-phase energy is applied to the neutralizing condensers C1 and C2, but by, employing this four-electrode holder the phase of the energy fed through the crystal at terminal 5 is opposite to that from terminal 8.

In Fig. 4 there is shown a modification of the system of Fig. 1. In this modification it is assumed that the ,I. F. energy produced in the output circuit of a first converter. is to be reduced to a-second, and lower, I. F. value. In other words, the receiver may be a superhetero dyne of the triple detection type. In that case, the converter I00 has its input grid I00 connected to the output circuit of the first I. F. network, while its plate has in circuit therewith the tuned primary circuit of the transformer ill. mary and secondary circuits of transformer III is to be tuned to the second I. F. value, and that the latter is substantially lower than the first I. F. value. The remainder of the converter Hill is well known to those skilled in the art. The cathode and first two cold electrodes are connected to provide a localoscillator network which produces oscillations whose frequency differs from the first I. F. by the second I. F. value. The numeral I06 designates the tank circuit of the oscillator section of converter Hill.

The numeral l0l denotes a stage of amplification for the second I. F. energy, and those skilled in the art are fully aware of the construction of that amplifier stage. The output electrodes of amplifier l0] are coupled to the crystal filter H3 by the transformer H2 whose primary and secondary circuits are each tuned to the second I. F. value. It will be understood that the primary and secondary circuits of each of transformers Hi and H2 are to be so constructed and arranged that each of thempasses the entire frequency range of the transmitted waves.

. The crystal H3 is of the type in which the inherent electrode capacities produce the proper degrees of neutralization. As explained prethe natural holder capacities can be adjusted by properly positioning the electrodes so as to obtain the desired capacities corresponding to C1 and C2 of Fig. 1. It is to be understood thatthe crystal filter H3 is such a filter network. As shown in connection with Fig. 1, the input electrodes I30 and i3! are connected to theopposite sides of the tuned secondary circuit fllZf,

' the lowerinput electrode 13! of the crystalifilter being grounded. The output electrodes [32' and I33 of the crystal filter are connected the respective input grids I20 and, [2V oith e pair of triodes disposed in detector tube-1102; These triodes of tube I02 are connected 'aslin finite-impedance diode detectors. Thus, the cathodes of both triodes are connected to ground does not need adjustment with regard to neu-j tralization.

It will be understood that each of. the pri- I through separate load resistors I20 and I2 I, each of these load resistors being bypassed for I. F. I currents but not for modulation voltage, where- 'by degenerative feedback of modulation voltage source, and the modulation is derived from across the resistor loads I20 and I2I. The modulation voltage is fed in push-pull fashion to the control grids I31 and I38 of the twin triode tube I03.

Tube I03 hastwo separate triode sections whose cathodes are connected in common to ground through the customary bypassed grid biasing resistor I38. The control grids I31 and I38 of the triodes are returned to ground through respective resistors I30 and I40. Grid I 31 is connected to the cathode end of load resistor I20 by condenser I, while grid I38 is connected to the cathode end of load resistor I2I by condenser I42. The transformer H4 islocated in the Common output circuit of modulation amplifier I03. The transformer II4 has two primary windings H5 and I48. Winding I45 is located in circuit with plate I43 of tube I03. Plate I44 of the tube I03 is connected by lead I50 to terminal I5I of double pole-double throw switch S. The latter is schematically represented. Winding I45 may be connected in either push-pull or pushpush relation with winding I45 by suitable adjustment of switch S. When terminals I53 and I54 of winding I45 are connected to respective terminals I52 and I5I of switch S, the +3 terminal feeds directly into the junction of windings I45 and I45 and the push-pull connection exists. When switch S is adjusted to connect terminals I55 and I55 of winding I45 to respective terminals I52 and I5I, then the windings I45 and I48 are in push-push relation. The secondary winding I" of transformer II4 feeds a pair'of separate output jacks H5 and H5. For phase modulation reception transformer H4 is connected in the push-pull connection by means of switch S, and the modulation output is taken from the jack I I5.

, For amplitude modulation reception switch S is actance-simulation tubes I04 and I05. These re-- actance tubes are connected in a push-pull circuit with their plate circuits common so that the reactive effects aid in varying the tuning of 'oscillator I05. This type of push-pull reactance circuit has been described in my application Serial No. 311,074, filed December 27, 1939, granted April 7, 1942, as U. S. Patent No. 2,278,429. The tuned circuit I08 is resonated to the frequency of circuit I05 so that the tuned circuit I08 has a resistive effect. Condenser H8 is adjusted to have a high reactance in comparison to the resistive effect of tuned circuit I08. This results in a current which is capacitive, and the voltage drop across the tuned circuit I08 is in phase with the to the grid of tube I04. Amplification of this points of positive potential of a direct current the plate circuit of tube I04, because the plate current is caused to lag the applied voltage.

The voltage fed to the grid of tube I from tuned circuit I08 is shifted in phase 180 degrees with respect to that fed to tube I04. This causes the grid voltage to lead the plate voltage, with the result that the plate circuit acts as a capacity since the current leads the applied voltage. Hence, when differential control potentials are fed from the time constant circuits I08-I I0 the inductance of tube I04 is caused to decrease at the same time that the capacity of tube I05 is caused to decrease. The resultant eflects of the two tubes I04 and I05 therefore aid, and in this case would increase the frequency of oscillation of tuned circuit I08. 7

As shown in Fig. 4, the automatic frequency control potentials are derived from the cathode ends of resistors I and HI. The time constant resistors I00 and I00 are included in the respective automatic frequency control leads to the current and, therefore, feeds a lagging voltage lagging voltage produces an inductive effect in control grids of tubes I04 and I05. The coil of circuit I08 is a split coil. The AFC leads are connnected through resistors I00 and I08. to the adjacent ends of the split windings of circuit I 08. The plates of tubes I 04 and I05 are connected in common to the positive terminal of a source of direct current through coil I01, and the high potential side of circuit I05 is connected to the plate side of choke I0'l through the coupling condenser I0I'. It is believed that the operation of the circuit shown in Fig. 4 will be clear from the description given in connection with Fig. 1.

. The receiver modification shown in Fla. 5 is a circuit performing the same functions as the circuit of Fig. 4. In this circuit the crystal filter network is designated by numeral 20I, and the output electrodes of that crystal filter feed the diodes 202 and 203 directly. In this circuit the transformer 200 feeds the second I. F, energy from the converter, while the diode anodes are connected to each other through series resistors 204 and 205. The junction of these latter resistars is connected to the Junction of cathode resistors 204' and 205'. Resistors 201 and 208 are connected in shunt across resistors 204' and 205',

the junction of the resistors 20! and 208 being established at ground potential.

Eachof resistors 204' and 205' is bypassed for I. F. currents, and the junction of these two resistors is connected to ground through the condenser 205. It will, therefore be seen that the ground on the mid-point of the diode resistors is made through condenser 205 and the mid-point of resistors 20'! and 208. This method of grounding the diodes removes the permanent bias, which is present on the diode resistors, from the grids of the reactance-simulation tubes 220 and 22I. In the normal in-tune, or balanced, condition of the diode currents there will be no current flow through resistors 201 and 208.- Therefore, there will be no bias fed to the grids of tubes 220 and 22I. If the ground point was placed at the mid-point of the diode resistors, or condenser 205 were short-circuited, the direct current potential across the diode resistors would be fed to the grids of tubes 220 and 22I to give them a permanent bias. I

Amplifiers 208 and 2 I0 combine the modulation output in push-pull for phase modulation reception, and the phase modulation voltage is taken off from jack II I. Amplifiers 2I2 and 2I3 combine the detected outputs of diodes 202 and 203 in push-push relation, and make them available on Jack 2I4.' For amplitude modulation recepof tubes 220 and 22l.

cycle range. ter 30! are connected to opposite sides of the section the equalization of the lower modulation frequencies is produced by means of series condensers 222-and 223 and shunt resistors 224 and 225. The automatitc frequency control potentials are fed to the reactance tubes 220 and 22l through resistors 2l3 and 2" respectively and condensers 2 l8 and 2 l 9 which form time constant circuits to remove modulation potentials from the reactance tubes. These reactance tubes are connected in this case through resistance-capacity phase shifters 240, 2 and 250, 25!.

Thus, the plates of tubes 220 and 22! are connected in common to the positive terminal of the direct current source through the choke 101. The plate of tube 22| is connected to the grounded end of its cathode biasing resistor through the phase shifter path 240, 2. In the same way. the plate .of tube 220 is connected to its grounded cathode biasing resistor through the phase shifter 25l, 250. The automatic frequency control bias is applied to the control grids of each Thus, the time constant resistor H1 is connected to the lower end of grid leak resistor 24l', while the time constant resistor 216 is connected to the end of the phase shifter resistor 250 adjacent condenser US. It will be understood that the plates of tubes 220 and 22! are connected across the tank circuit I in the same manner as shown in connection with Fig. 4. a v

Tube 21" produces an inductive effect in its plate circuit, since resistor 240 is adjusted to be large in comparison to the reactance of condenser 2 so that the grid voltage lags the voltage ap- ,i of crystal filter 30l connected to a pair of voltage-doubling diodes 302 and 303.

Thus, the transformer 300- may have its primary and secondary circuits each tuned to the operating intermediate frequency, and it is to be understood that this value may be in .the megacycle or kilo- The input electrodes of crystal filondary circuit 300', the low potential side of the latter being at ground potential. Tube 302 may be a 6H6 type of double diode. In that case the anode of one diode and the cathode of the other are connected together, and are coupled to the output electrode 306 through the coupling condenser 301. In the same way,- the cathode of one diode and the anode of the other diode of tube 303 are connected in common to the output electrode 306' of the crystal filter through coupling condenser 301'.

The cathode 3l0 of the diode whose anode is connected to coupling condenser 301 is connected to the cathode 3 of the diode whose anode is connected to coupling condenser 301'. In the same way, the anode 320 of the diode whose cathode is connected to the coupling condenser 301 is connected to the high potential end of output resistor 304. The anode 32l of the diode whose cathode is connected to coupling condenser 301' is connected to the high potential end of load resistor 305. The junction of resistors 304 and 305 is grounded, and each of these load resistors is bypassed for I. F. currents... It will be noted that in Fig. 6 the separate output terminals of "resistors 404 and 406 arranged in series.

the c ystal fiiter'feed the diodes 302 and 303 which are connected individually in voltage-doubling circuits. The detected outputs appear across resistors 304 and 305.. Phase modulation voltage, or AFC potential, is obtained from the difierentially connected resistors 304 and 305. It will be seen that amplitude modulation can be obtained from these same terminals by the method shown in Fig. 4. Here, again, the cristal filter 30l is of the type shown in Fig. 1, except that the capacities C1 and C: are provided by the natural crystal holder capacity as previously explained. v a

The arrangement shown in Fig. '7 is a modification of that shown in Fig. 6. Here the self-neutralized type of crystal 30I is arranged to feed a pair 'of diodes 402 and 403 having cathode load The junction of these cathode resistors is connected to the junction of series resistors 401 and 400, the latter being connected between the output electrodes 306 and 306' of the crystal filter. The

cathode end of resistor 405 is grounded, while each of the cathode resistors is bypassed for I. F..

currents. The PM voltage output and the AFC potential are taken off from the cathode end of resistor 404. In other words, the diodes 402 and 403 are connected differentially. This type of connection is convenient for single-ended amplification of the PM voltage output, and for the application of the automatic frequency control potentials to a single reactance-simulation tube.

While I have indicated and described several systems for carrying my invention into efl'ect; it will be apparent to one skilled in the art that my invention is by no means limited to the particular lated carrier waves, a detector network comprising at least two demodulator tubes having respective load resistors arranged in the space'currentpaths of the demodulator tubes, a crystal filter network comprising a piezo-electric crystal, a pair of opposed electrodes having one end of said crystal located therebetween, said electrodes being connected to opposite sides of said input circuit, a. second pair of electrodes having the opposite end of said crystal located therebetween, the electrodes of said second pair being connected to respective input electrodes of said demodulator tubes, and the electrodes of said crystal filter being arranged to provide sufllcient self-neutralization to produce conversion of the phase modulated carrier waves into corresponding. am-

located therebetween, said electrodes being con- I nected to opposite sides of said input circuit, a second pair of electrodes having the opposite end of said crystal located therebetween, the electrodes of said second pair being connected to respective input electrodes of said demodulator tubes, and the electrodes of said crystal filter being arranged to provide suiiicient self-neutralization to produce conversion of the phase modulated carrier wavesinto corresponding amplitude modulated carrier waves, means for deriving modulation voltage from across said load resistors, and additional means arranged in pushpush relation for deriving modulation voltage from across said load resistors when amplitude modulated carrier waves are applied to said crystal filter input circuit.

3. In combination with a phase modulated carrier input circuit, a demodulator network providediwith push-pull arranged input circuits, a crystal filter network for converting phase modulated waves into amplitude modulated waves suitable for detection, said crystal filter comprising a piezo-electric crystal and two pair oi holder electrodes, said crystal having its opposite ends disposed between the spaced elec- .trodes of each of said two pair of holder elecand the second electrode 01' said second pair of Y electrodes to produce over-neutralization.

4. In combination with a phase modulated carrier input circuit, a demodulator network provided with push-pull arranged input circuits,

a crystal filter network, for converting phase modulated waves into amplitude modulated waves suitable for detection, comprising two pair of holder electrodes, a piezo-electric crystal having its opposite ends disposed between the spaced electrodes 01' each of said two pair of holder. electrodes, means connecting the spaced electrodes of one or said pair of holder electrodes to opposite sides 0! said phase modulation input circuit, means for connectinlthe spaced 6166-1 trodes oi the second pair to respective ones of said demodulator input circuits, means provid-,

ing sufilcient capacity between two of said holder electrodes to produce under-neutralization, additional means providing suiiicient capacity between one 01 said two electrodes and a third of the holder electrodes to produce over-neutralization, and additional means to provide the de- I modulation of amplitude modulated carrier waves upon the application of such modulated waves to said crystal filter input circuit.

5. In combination, a modulated carrier wave input circuit'having one side thereof at an 'invariable potential, two demodulator tubes having respective load resistors arranged in the space current paths of the demodulator tubes, a crystal filter network comprising a piezoelectric crystal, a pair 01' opposed electrodes having one end of said crystal located therebetween, said electrodes being connected to opposite sides of said input circuit, a second pair of electrodes having the opposite end of said 70 crystal located therebetween, means to couple the electrodes 01' said second pair to respective input electrodes of said demodulator tubes, and separate external neutralizing condensers connected from each ofthe second pair of electrodes 7 to the electrode 01 the first pair connected to the high potential side of said input circuit.

6. In combination with a phase modulated carrier input circuit, a demodulator network provided with a pair of diodes having push-pull arranged input circuits, a crystal filter network for converting phase modulated waves into amplitude modulated waves-suitable tor detection, said crystal filter comprising a piezo-electric crystal having its opposite ends disposed between the spaced el ectrodes of each of two pair of holder electrodes, means connecting the spaced electrodes of one of said pair of holder electrodes to opposite sides of said phase modulated input circuit, one side of said input circuit being grounded, means for c onnecting the spaced electrodes of the second pair to respective ones of said demodulator input circuits, a condenser connected between one of the second pair of electrodes and the ungrounded electrode of said first pair for providing sufilcient capacity to produce under-neutralization, a second condenser connected between the remaining electrodeof the second pair and said ungrounded electrode providing sufficient capacity to produce over-neutralization.

7. In combination with a source 01' phase modulated carrier waves, a crystal filter network for converting the phase modulated carrier waves into amplitude modulated waves, a pushpull diode detector network for deriving modulation voltage from the converted modulated carrier waves, said source consisting of a resonant input circuit, saidfilter networkcomprising a piezo-electric crystal, a pair of input electrodes and a pair of output electrodes, said crystal having its opposite ends located between said two pairs of electrodes, said input pair being connected to opposite sides of said input circuit, means coupling said detector network to said output electrodes, and separate condensive coupling from each of the output electrodes to a common side 01' said input circuit.

8. In combination with a phase modulated carrier input circuit, a demodulator network provided opposed diodes having push-pull arranged input circuits, a crystal filter for converting phase modulated waves into amplitude modulated waves suitable for detection. said crystal filter comprising a piezo-electric crystal having its opposite ends disposed between the spaced electrodes of each of two pair oi! holder electrodes, means connecting the spaced electrodesof one of said pair of holder electrodes to opposite sides or said phase modulated input circuit, meansgrounding one side or the input circuit, means for connectin the spaced electrodes of the second pair.to respective ones of said demodulator input circuits, meansproviding sufiicient capacity between an electrode of the second pair and the ungrounded electrode 01 the first pair to produce under-neutralization,

additional means providing sufiicient capacity between the other electrode of the second pair and the ungrounded electrodeto produce overneutralization, and additional means to provide the modulation of amplitude modulated carrier waves upon the application of such modulated waves to said crystal filter input circuit.

9. In combination, a tuned input circuit reso' nant to applied phase modulated carrier waves,

a detector network comprising at least two diode demodulator tubes having respective load re.- sistors arranged in series in the space current paths thereofia pair oi resistors arranged in series between the diode anodes, the junctions or the load resistors and said pair being connected,

' a crystal filter network comprising a piezo-eleccrystalfllter being arranged to provide suflicient self-neutralization to produce conversion or the phase modulated carrier waves into corresponding amplitude modulated carrier waves, resistive means in shunt with the load resistors for deriving modulation voltage from across said load I resistors, and additional means connected tosaid last resistive means for deriving modulation voltage from across said load resistors when amplitude modulated carrier waves are applied to said crystal filter input circuit.

' MURRAY G. CROSBY. 

